You need to link your program using the -threaded switch. (NOTE: previously it was necessary to compile all code, including libraries, with the -smp switch, this is no longer the case. The -smp flag is now a synonym for -threaded).

Run the program with +RTS -N2 to use 2 threads, for example. You should use a -N value equal to the number of CPU cores on your machine (not including Hyper-threading cores).

Concurrent threads (forkIO and forkOS) will run in parallel, and you can also use the par combinator and Strategies from the Control.Parallel.Strategies module to create parallelism.

Use +RTS -sstderr for timing stats.

Links to related work on parallel and distributed Haskell (many based on GHC)

Problems with GHC implementation before 6.6.1

There are critical differences between the description in the paper
"Asynchronous exceptions in Haskell by Simon Marlow, Simon Peyton
Jones, Andy Moran and John Reppy, PLDI'01." the implementation in GHC
6.4 and GHC 6.6 today.